The present invention relates to a rotor assembly for an electrical machine and a P.M. (Permanent Magnet) motor comprising such a rotor assembly. In general, the invention relates to the field of brushless D.C. and P.M. synchronous motors comprising permanent magnets which can be configured to include an interior rotor surrounded by a stator (Innenläufermotor) or on outer rotor configuration (Auβenläufermotor). Electrical machines having an interior rotor include a rotor assembly which is mounted on the shaft and one or more permanent magnets as well as a stator assembly, such as a stator laminated from a number of metal sheets, comprising windings. The rotor assembly is coaxially inserted into the stator assembly. In electrical machines having outer rotors the rotor assembly surrounds the stator.
FIG. 5 schematically shows the general design of an electrical machine, comprising a housing 114, in which are included the stator assembly 118, the rotor assembly 116 as well as bearings 126, 128 for rotatably supporting the rotor assembly. The stator assembly 118 comprises sheet metals 155 and windings 160 and defines an interior cavity into which the rotor assembly 116 can be inserted. The rotor assembly 116 includes the shaft 110, a yoke 112 and permanent magnets 122. The bearings 126, 128 for the rotor assembly can be integrated into a flange 124 of the motor housing 114.
More particularly, the present invention relates to a rotor assembly for an electrical machine, comprising a body of generally cylindrical shape having an inner opening for coaxially mounting the body on a shaft, and permanent magnets embedded in said body.
Rotors including embedded magnets have been generally known and described in the art. A rotor configuration having a multi-pole “spoke” design with radially extending embedded magnets, enclosed by a retaining ring is shown e.g. in “Design of Brushless Permanent-Magnet Motors” J. R. Hendershot Jr. and T J E Miller, Magna Physics Publishing and Clarendon Press, Oxford, 1994. As shown therein, it is known to have a rotor body with embedded, radially extending magnets which are protected by a ring or tube surrounding the rotor body. The rotor body in which the magnets are embedded has the function of a yoke.
A common form of burried magnets is shown in EP 0 641 059 B1 and EP 0 691 727 B1. These references show a plurality of magnets which are inserted into slots which are formed in the outer rotor surface. With the design disclosed in these references, stray flux is created which passes through the rotor back-iron close to the shaft. Accordingly, magnet energy dissipated in this area can't be used efficiently. Document DE 101 00 718 A1 discloses a similar design wherein the permanent magnets are inserted into slots in the rotor body which are closed at the outer rotor surface. Thereby, the rotor is divided into wedges which need to be mounted on a frame.
WO 00/57537 describes a multipole, permanent-magnet rotor for a rotating electrical machine, manufactured with embedded magnets in a so-called “flux-concentrating style”. In the rotor configuration shown in this reference permanent magnets are designed as flat cubes which are arranged in a direction radial to the rotor axis in groove-like gaps between yoke sections fixed in the rotor body. For easily mounting the magnets this document proposes to divide a yoke into two adjacent half-yokes of two poles, wherein magnets are arranged there between to form a pole element which can be fixed independently on the rotor body.
EP 0 872 944 A1 shows another design of a rotor having embedded permanent magnets wherein the magnets are arranged in a radial direction or parallel to a radial direction of the rotor body.
A similar design is shown in EP 0 803 962 B1. The rotor is made of a ferromagnetic material body defining a plurality of radially by extending slots in which are engaged the permanent magnets so as to provide the desired numbers of poles for the rotor. The slots for receiving the permanent magnets are designed with a bridge at the outer periphery of the rotor body and an opening at the inner diameter of the rotor body so that the permanent magnets can be inserted and held in said slots. Trapezoidal elements of the rotor body which are separated by the radially extending slots will form the poles of the rotor.
Further patent documents showing a rotor having embedded magnets are GB 1,177,247, EP 0 955 714 A2, and U.S. 2002/0,067,096 A1.
The preferred application of the rotor assembly according to the present invention is in a brushless D.C. motor or P.M. synchronous motor. Such motors can be used in a wide range of applications such as spindle motors for disc drives, electrical motor power assisted systems in automobiles, e.g. steering and braking systems, power tools and many other applications.
With the radial arrangement of the permanent magnets, as shown and described in the prior art, a problem may arise in that the regular arrangement of the permanent magnets produces a cogging torque effect which is disadvantageous in the operation of the motor. In a rotor assembly having surface-magnets, it is known to provide a skewed magnet arrangement to avoid abrupt switching between phases and thus to reduce a cogging torque, as disclosed e.g. in the above mentioned reference of Hendershot and Miller. Skewed magnetization of the rotor poles, however, creates an axial component of the magnetic force and thus a loss of torque. Further, the problem of the cogging torque produced during operation of the electrical motor has not yet been solved satisfactorily for rotors having embedded magnets of the type described above.
It is therefore an object of the present invention to provide a rotor assembly having embedded magnets which can improve the efficiency of the electrical machine and, in particular, eliminate or reduce the cogging torque.